The present invention relates, in general, to oscillating timing signals and, more particularly, to circuits and methods for generating and calibrating oscillating timing signals.
Oscillators play essential roles in communication and information transfer. They provide clock signals to the circuits communicating with each other. A central part of an oscillator is usually a piece of quartz crystal. A voltage potential is applied across the crystal, causing mechanical vibration at a resonant frequency. The thickness of the crystal determines its nominal oscillating frequency. The crystal oscillating frequency also depends on other factors such as, for example, temperature, the angle of the cut, load capacitance, etc. A voltage controlled crystal oscillator (VCXO) seeks to control the frequency of the oscillator by adjusting the load capacitance of the crystal in the oscillator. A temperature controlled oscillator, which is often referred to as an oven controlled oscillator (OCXO), seeks to increase the frequency stability of the oscillator by adjusting the temperature of the crystal.
When two circuits such as, for example, a base station and a user terminal in a wireless communications network, communicate with each other, the clock signals in the two circuits are preferably synchronized with each other. This can be achieved by using the clock signal in one circuit, e.g., the base station, as the master signal, and synchronizing the clock signal in the other circuit, e.g., the user terminal, to master signal. The clock signals of different circuits, e.g., different base stations in a wireless communication network, can also be synchronized using a common signal, e.g., a public switched telephone network (PSTN) signal, as the master clock signal. Alternatively, the clock signal from a global positioning system (GPS) satellite can serve as the master signal, wherein the clock signals of all base stations in a wireless communication network are synchronized to the GPS signal. That the timing be synchronized between base stations is especially critical during soft hand-offs, where a mobile terminal communicates with more than one base station at the same time.
Because of naturally occurring variations in crystal oscillation frequencies, synchronizing clock signals of different circuits in communication with one another is an ongoing process. For example, in order to synchronize the clock signal in a base station to a master signal from a PSTN or a GPS satellite, the base station must first be able to receive the master signal. In operation, however, the wireless communication base station may temporarily lose contact with the PSTN or GPS satellite, e.g., due to inclement weather conditions or equipment failure. During this xe2x80x9choldover period,xe2x80x9d the base station cannot synchronize its clock signals to a master signal from the PSTN, or from the GPS satellite.
Accordingly, it would be advantageous to provide synchronization circuits and methods for both generating a clock signal and then for calibrating the clock signal during a period when an oscillating clock circuit is isolated from an outside master synchronization signal. For example, it would be advantageous to have a process for calibrating the clock signal of a base station in a wireless communications network during a holdover period, thereby keeping the base station substantially synchronized with other base stations in the network. Further, it is desirable for the calibration process to take different factors that affect the crystal oscillation frequency into consideration, such as temperature changes and aging.
In accordance with a general aspect of the invention, a process for generating and calibrating an oscillator clock signal is provided, which enables the clock signal to remain substantially synchronized with clock signals of other common circuit entities, e.g., in a communications network, even when the oscillator loses contact with a master synchronization signal during a holdover period. As used herein, the term xe2x80x9ccalibrationxe2x80x9d is to be construed broadly, including without limitation the process of generally synchronizing an oscillator clock signal to a master synchronization signal, as well as the process of calibrating the oscillator clock signal when it is not in contact with the master synchronization signal.
In accordance with a more specific aspect of the invention, the calibration process takes certain physical factors that affect the crystal oscillation frequency into consideration and generates a crystal oscillation frequency function describing the relationship between the crystal oscillation frequency and those factors. For example, the crystal oscillation frequency generally depends on the thickness of the crystal, the load capacitance of the crystal oscillator, the temperature of the crystal, and the age of the crystal. Thus, where the crystal oscillator does not have a variable capacitance load, and the dimension of the crystal in the oscillator does not change over time, temperature and time are the two parameters that may significantly affect the crystal oscillation frequency.
Crystal physical models and experiments show that the temperature effect and the time effect on crystal oscillation frequency are substantially independent of each other. Thus, the crystal oscillation frequency function can be expressed as the sum of two single variable sub functions. One sub function, having temperature as the single variable and referred to as a xe2x80x9ctemperature function,xe2x80x9d describes the relationship between the crystal oscillation frequency and the temperature.
In one embodiment, the temperature function is expressed as a polynomial of temperature, wherein the order of the polynomial depends on the crystal model and the desired accuracy.
Other base functions can be used in other embodiments to express the temperature function, e.g., wavelets or a Fourier transformation. Another option is to use an orthogonal basis instead of a transformation function, e.g., wherein statistical data based on prior temperature measurements of a quantity of crystals is used. When a polynomial is used, higher orders of the polynomial give higher accuracy, but are more complex and requires more memory and calculation time. The temperature coefficients in the polynomial are preferably continuously estimated and reevaluated during the periods when the oscillator circuit has access to a master synchronization signal. This continuous estimation and reevaluation process keeps the temperature coefficients updated, so as to accurately describe the relationship between the crystal oscillation frequency and temperature.
The other sub function, having time as the single variable and referred to as a xe2x80x9cdrift function,xe2x80x9d describes the relationship between the crystal oscillation frequency and time. The drift function reflects the aging effect of the quartz crystal. Typically, the drift function is a logarithmic function of time with a time constant on the order of weeks, whereas the duration of a holdover period is typically on the order of minute, or hours. During each holdover period, a first order polynomial, i.e., a linear function, with appropriate aging coefficients can express the aging effect of the crystal with sufficient accuracy. Like the temperature coefficients in the temperature function, the aging coefficients in the drift function are preferably continuously estimated and reevaluated during the periods when the oscillator circuit has access to a master synchronization signal. This continuous estimation and reevaluation process keeps the aging coefficients updated, so as to accurately describe the relationship between the crystal oscillation frequency and time. Again, other base functions can be used to express the aging function, e.g., the use of wavelet functions would be well-suited for modeling the aging function, but would add complexity to the system design.
Once the temperature and aging coefficients are known, a crystal oscillation frequency function describing the dependence of the crystal oscillation frequency on temperature and time is determined. During a holdover period, the crystal oscillation frequency function predicts the frequency of the oscillator clock signal. The prediction is compared with a target frequency of the oscillator to generate an error signal, which is also referred to as a calibration signal. A synthesizer is then used to generate a stable clock signal based upon the oscillator output and error signal. The synthesizer is incorporated as part of the existing circuitry for synchronizing the clock signal of the circuit to the master synchronization signal during the lock-in periods, thereby simplifying the calibration process implementation and minimizing the circuit complexity.
Other aspects and features of the invention will become apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings.